Environmental concerns and the need for fuel conservation has spurred the development of new hybrid propulsion systems for vehicles. Hybrid electric vehicle (HEV) powertrains for example, typically include electric traction motors, high voltage electric energy storage systems, and modified transmissions. Electric energy storage systems include batteries and ultra capacitors. Primary power units for these systems may include spark ignition engines, compression ignition direct injection (e.g., diesel) engines, gas turbines and fuel cells.
HEV powertrains are typically arranged in series, parallel or parallel-series configurations. With parallel-series arrangements, multiple motors operating in multiple operating modes sometimes require the use of several gear sets to effectively transmit power to the traction wheels. As a result, HEV powertrains often possess considerable effective inertia at the wheels compared to conventional ICE powertrains. This is due in part to the potentially large inertia of the hybrid motor devices, as well as the significant gearing from motor to wheels that is often employed.
Powertrains possessing relatively high effective inertias such as those of HEVs, result in certain problems that require solutions. For example, the application of braking force to the vehicle's traction wheels during a sudden braking event, may result in a very rapid angular momentum change in the powertrain. Specifically, a rapid deceleration of the traction wheels during braking results in a counter-torque being transmitted from the traction wheels back through the driveline. Because many of the components connected in the driveline have relatively large effective inertias at the wheels, the counter-torque produced by the braking event can produce relatively high reactive torque levels in the powertrain. This reaction torque is transmitted through the gearing mechanisms to the transmission housing, and can have deleterious effects on powertrain and driveline components, particularly under sudden conditions, such as when the vehicle's ABS system is activated.
The problems described above can be exacerbated by automated control of the vehicle's braking system, as occurs when the vehicle's ABS is actuated, since the ABS is capable of cycling or “pulsing” the brakes at a frequency much higher than the driver. Because of the large inertia of the HEV's powertrain and highly geared electric motor, pulsing the brakes can excite the natural vibration frequencies of the vehicle or certain of its mechanical subsystems, such as the driveline and powertrain. For example, ABS excitation at certain frequencies can excite the roll mode of the engine block and transmission case on the engine mounts. Depending on the mounting configuration, excitation of this roll mode can also excite engine block modes such as fore/aft or lateral motion. The excitation described above can occur for either of two reasons. First, the large inertias and high gear ratios characteristic of HEV's shift some of the natural frequencies down to a range where they can be excited by the ABS. Second, the ability of the braking system to excite additional modes may be amplified because of the large inertias and high gear ratios. ABS excitation of the natural frequencies of the vehicle's mechanical systems can impose undesirable levels of stress on driveline and powertrain components, increasing the likelihood of noise, vibration, harshness, and even component degradation.
Accordingly, there is a need in the art for a system for avoiding or reducing pulsing of a vehicle's brakes at certain natural frequencies of the vehicle's mechanical components and subsystems. The present invention is intended to satisfy this need.